Transition metals such as molybdenum and tungsten and generally the metals in transition group VI of the periodic table of elements have important applications in sophisticated areas of modern technology such as high speed impeller blades in turbines and aircraft engines operating at high temperatures and in miniature electrical components. Because these metals will form surface oxides at room or elevated temperatures and because the oxides can impair the use of articles formed of these metals, it has often been necessary to encapsulate or coat such metals with a protective metal plate.
Techniques are known for plating these transition metals, but the provision of a protective metal plate is complicated by the chemically stable but physically unstable state of some of the metal oxides and the tendency of some of these chemically stable oxides to migrate to the surface if they remain in the interface between the transition metal and the metal plate. The migration of the oxides promotes further oxidation of the transition metal substrate and deterioration of the surface qualities of the plate layer as by a loss of surface smoothness or separation of the protective plate from the metal substrate. A failure of vital components formed of the plated transition metal may thereby ultimately occur.
One prior art method for coating molybdenum is disclosed in U.S. Pat. No. 3,386,896 and seeks to protect the molybdenum with a gold layer. An initial, thin gold strike is electroplated on hydrated molybdenum oxides followed by a subsequent reduction of the molybdenum oxides with hydrogen in a furnace at elevated temperatures to drive off the oxygen. Finally, a thicker gold layer is plated onto the initial layer to encapsulate the molybdenum against further oxidation. The process employs a sequential series of reaction processes that require a separate reduction of molybdenum oxides after the initial formation of a thin, electroplated gold strike and resultant exposure of the thinly plated molybdenum to the atmosphere and possible oxidation between steps.